High-resolution cryo-SEM allows direct identification of F-actin at the inner nuclear membrane of Xenopus oocytes by virtue of its structural features

Nuclear actin filaments - a historical perspective

The view on nuclear filaments formed by non-skeletal β-actin has significantly changed over the decades. Initially, filamentous actin was observed in amphibian oocyte nuclei and only under specific cell stress conditions in mammalian cell nuclei. Improved labeling and imaging technologies have permitted insights into a transient but microscopically apparent filament network that is relevant for chromatin organization, biomechanics of the mammalian cell nucleus, gene expression, and DNA damage repair. Here, we will provide a historical perspective on the developing insight into nuclear actin filaments.

ZC3HC1 Is a Novel Inherent Component of the Nuclear Basket, Resident in a State of Reciprocal Dependence with TPR

The nuclear basket (NB) scaffold, a fibrillar structure anchored to the nuclear pore complex (NPC), is regarded as constructed of polypeptides of the coiled-coil dominated protein TPR to which other proteins can bind without contributing to the NB's structural integrity. Here we report vertebrate protein ZC3HC1 as a novel inherent constituent of the NB, common at the nuclear envelopes (NE) of proliferating and non-dividing, terminally differentiated cells of different morphogenetic origin. Formerly described as a protein of other functions, we instead present the NB component ZC3HC1 as a protein required for enabling distinct amounts of TPR to occur NB-appended, with such ZC3HC1-dependency applying to about half the total amount of TPR at the NEs of different somatic cell types. Furthermore, pointing to an NB structure more complex than previously anticipated, we discuss how ZC3HC1 and the ZC3HC1-dependent TPR polypeptides could enlarge the NB's functional repertoire.

Actin depolymerization disrupts karyosphere capsule integrity but not residual transcription in late oocytes of the grass frog Rana temporaria

Late diplotene oocytes are characterized by an essential decrease in transcriptional activity. At this time, chromosomes condense and form a compact structure named a karyosphere. The karyosphere of grass frogs Rana temporaria is surrounded by a fibrillar karyosphere capsule (KC). One of the main protein constituents of R. temporaria KC is actin. In this study, we used antibodies against different actin epitopes to trace different forms of actin in the KC. We also investigated the effect of F-actin depolymerization on the oocyte nuclear structures and transcription of chromatin DNA and rDNA in the amplified nucleoli. It was determined that disruption of actin filaments leads to chromosome shrinkage, nucleoli fusion, and distortion of the KC structure, but does not inhibit residual transcription in both the karyosphere and the nucleoli.

Mucorales spores induce a proinflammatory cytokine response in human mononuclear phagocytes and harbor no rodlet hydrophobins

Mucormycoses are life-threatening infections in immunocompromised patients. This study characterizes the response of human mononuclear cells to different Mucorales and Ascomycota. PBMC, monocytes, and monocyte derived dendritic cells (moDCs) from healthy donors were stimulated with resting and germinated stages of Mucorales and Ascomycota. Cytokine response and expression of activation markers were studied. Both inactivated germ tubes and resting spores of Rhizopus arrhizus and other human pathogenic Mucorales species significantly stimulated mRNA synthesis and secretion of proinflammatory cytokines. Moreover, R. arrhizus spores induced the upregulation of co-stimulatory molecules on moDCs and a specific T-helper cell response. Removal of rodlet hydrophobins by hydrofluoric acid treatment of A. fumigatus conidia resulted in enhanced immunogenicity, whereas the cytokine response of PBMCs to dormant R. arrhizus spores was not influenced by hydrofluoric acid. Scanning electron micrographs of Mucorales spores did not exhibit any morphological correlates of rodlet hydrophobins. Taken together, this study revealed striking differences in the response of human mononuclear cells to resting stages of Ascomycota and Mucorales, which may be explained by absence of an immunoprotective hydrophobin layer in Mucorales spores.

Analysis of nuclear actin by overexpression of wild-type and actin mutant proteins

Compared to the cytoplasmic F-actin abundance in cells, nuclear F-actin levels are generally quite low. However, nuclear actin is present in certain cell types including oocytes and under certain cellular conditions including stress or serum stimulation. Currently, the architecture and polymerization status of nuclear actin networks has not been analyzed in great detail. In this study, we investigated the architecture and functions of such nuclear actin networks. We generated nuclear actin polymers by overexpression of actin proteins fused to a nuclear localization signal (NLS). Raising nuclear abundance of a NLS wild-type actin, we observed phalloidin- and LifeAct-positive actin bundles forming a nuclear cytoskeletal network consisting of curved F-actin. In contrast, a polymer-stabilizing actin mutant (NLS-G15S-actin) deficient in interacting with the actin-binding protein cofilin generated a nuclear actin network reminiscent of straight stress fiber-like microfilaments in the cytoplasm. We provide a first electron microscopic description of such nuclear actin polymers suggesting bundling of actin filaments. Employing different cell types from various species including neurons, we show that the morphology of and potential to generate nuclear actin are conserved. Finally, we demonstrate that nuclear actin affects cell function including morphology, serum response factor-mediated gene expression, and herpes simplex virus infection. Our data suggest that actin is able to form filamentous structures inside the nucleus, which share architectural and functional similarities with the cytoplasmic F-actin.

Ultrastructural imaging of endocytic sites in Saccharomyces cerevisiae by transmission electron microscopy and immunolabeling

Defining the ultrastructure of endocytic sites and localization of endocytic proteins in Saccharomyces cerevisiae by immunoelectron microscopy is central in understanding the mechanisms of membrane deformation and scission during endocytosis. We show that an improved sample preparation protocol based on high-pressure freezing, freeze substitution, and low-temperature embedding allows us to maintain the cellular fine structure and to immunolabel green fluorescent protein-tagged endocytic proteins or actin in the same sections. Using this technique we analyzed the stepwise deformation of endocytic membranes and immunolocalized the endocytic proteins Abp1p, Sla1p, Rvs167p, and actin, and were able to draw a clear ultrastructural distinction between endocytic sites and eisosomes by immunolocalizing Pil1p. In addition to defining the geometry and the fine structure of budding yeast endocytic sites, we observed associated actin filaments forming a cage-like meshwork around the endocytic membrane.

Imaging and elemental mapping of biological specimens with a dual-EDS dedicated scanning transmission electron microscope

A dedicated analytical scanning transmission electron microscope (STEM) with dual energy dispersive spectroscopy (EDS) detectors has been designed for complementary high performance imaging as well as high sensitivity elemental analysis and mapping of biological structures. The performance of this new design, based on a Hitachi HD-2300A model, was evaluated using a variety of biological specimens. With three imaging detectors, both the surface and internal structure of cells can be examined simultaneously. The whole-cell elemental mapping, especially of heavier metal species that have low cross-section for electron energy loss spectroscopy (EELS), can be faithfully obtained. Optimization of STEM imaging conditions is applied to thick sections as well as thin sections of biological cells under low-dose conditions at room and cryogenic temperatures. Such multimodal capabilities applied to soft/biological structures usher a new era for analytical studies in biological systems.

Novel electron tomographic methods to study the morphology of keratin filament networks

The three-dimensional (3D) keratin filament network of pancreatic carcinoma cells was investigated with different electron microscopical approaches. Semithin sections of high-pressure frozen and freeze substituted cells were analyzed with scanning transmission electron microscope (STEM) tomography. Preservation of subcellular structures was excellent, and keratin filaments could be observed; however, it was impossible to three-dimensionally track the individual filaments. To obtain a better signal-to-noise ratio in transmission mode, we observed ultrathin sections of high-pressure frozen and freeze substituted samples with low-voltage (30 kV) STEM. Contrast was improved compared to 300 kV, and individual filaments could be observed. The filament network of samples prepared by detergent extraction was imaged by high-resolution scanning electron microscopy (SEM) with very good signal-to-noise ratio using the secondary electron signal and the 3D structure could be elucidated by SEM tomography. In freeze-dried samples it was possible to discern between keratin filaments and actin filaments because the helical arrangement of actin subunits in the F-actin could be resolved. When comparing the network structures of the differently prepared samples, we found no obvious differences in filament length and branching, indicating that the intermediate filament network is less susceptible to preparation artifacts than the actin network.

The actin cytoskeleton in whole mount preparations and sections

In non-muscle cells, the actin cytoskeleton plays a key role by providing a scaffold contributing to the definition of cell shape, force for driving cell motility, cytokinesis, endocytosis, and propulsion of pathogens, as well as tracks for intracellular transport. A thorough understanding of these processes requires insight into the spatial and temporal organisation of actin filaments into diverse higher-order structures, such as networks, parallel bundles, and contractile arrays. Transmission and scanning electron microscopy can be used to visualise the actin cytoskeleton, but due to the delicate nature of actin filaments, they are easily affected by standard preparation protocols, yielding variable degrees of ultrastructural preservation. In this chapter, we describe different conventional and cryo-approaches to visualise the actin cytoskeleton using transmission electron microscopy and discuss their specific advantages and drawbacks. In the first part, we present three different whole mount techniques, which allow visualisation of actin in the peripheral, thinly spread parts of cells grown in monolayers. In the second part, we describe specific issues concerning the visualisation of actin in thin sections. Techniques for three-dimensional visualisation of actin, protein localisation, and correlative light and electron microscopy are also included.

Towards native-state imaging in biological context in the electron microscope

Modern cell biology is reliant on light and fluorescence microscopy for analysis of cells, tissues and protein localisation. However, these powerful techniques are ultimately limited in resolution by the wavelength of light. Electron microscopes offer much greater resolution due to the shorter effective wavelength of electrons, allowing direct imaging of sub-cellular architecture. The harsh environment of the electron microscope chamber and the properties of the electron beam have led to complex chemical and mechanical preparation techniques, which distance biological samples from their native state and complicate data interpretation. Here we describe recent advances in sample preparation and instrumentation, which push the boundaries of high-resolution imaging. Cryopreparation, cryoelectron microscopy and environmental scanning electron microscopy strive to image samples in near native state. Advances in correlative microscopy and markers enable high-resolution localisation of proteins. Innovation in microscope design has pushed the boundaries of resolution to atomic scale, whilst automatic acquisition of high-resolution electron microscopy data through large volumes is finally able to place ultrastructure in biological context.